$f(x) = \dfrac{ x + 5 }{ ( x + 5 )( x - 8 ) }$ What is the domain of the real-valued function $f(x)$ ?
$f(x)$ is undefined when the denominator is 0. The denominator is 0 when $x=-5$ or $x=8$ So we know that $x \neq -5$ and $x \neq 8$ Expressing this mathematically, the domain is $\{ \, x \in \RR \mid x \neq-5, \,x \neq8\, \}$.